Fluorescence Finds Photomask Defects

Using a fluorescent dye and an argon-ion laser, researchers at the University of Delaware can detect nanometer-size scratches in the polished surface of a photomask much more quickly than with atomic force microscopy.

Researchers use a fluorescent dye that sticks to photomask scratches in to find polishing defects that could hamper the mask's effectiveness.
A research team led by Mary Wirth and Daniel van der Weide designed the method while studying polishing methods for photolithography masks.

After cleaning the photomask, the researchers place a solution containing indocarbocyanine dye on the surface. The dye molecules stick to scratches and polishing marks.

After rinsing the surface and allowing it to dry, the researchers scan a 514-nm argon-ion laser over the surface. A charge-coupled device camera detects the fluorescence. Scratches and pits show the highest fluorescence intensity.

As chip features become smaller, photomasks must become more accurate or the chips will not work. Atomic force microscopy is the most common method of detecting defects, but it is slow. Wirth said that in one second, the fluorescence method can scan a region that would take an atomic force microscope 10 minutes.

Wirth said the method may be used for quality control of photomasks, but it is still too early to assess its usefulness. The technique may also be able to characterize surfaces for chemical separations.

"A region that is very adhering for dyes is also likely to be adhering for proteins," she said.